The use of chemical element analyses in the study of biostratigraphically barren sequences: an example from the Triassic of the central North Sea (UKCS)

Abstract

Abstract A detailed core sampling programme has been completed on biostratigraphically barren Triassic material from seven wells in the Central Graben of the central North Sea (UKCS). Sedimentological, petrographic and chemical element data from these Skagerrak Formation sequences are presented. The objective of the study was to examine the potential use of chemical stratigraphy in the stratigraphic correlation of sequences and the identification of differences in provenance using inductively coupled plasma-atomic emission spectrometry (ICP-AES). The cores examined were taken mainly from the upper parts of the preserved Skagerrak Formation and ranged in thickness from 21.6–130.5 m. Five major facies associations were identified: (1) meandering channels; (2) braided channels: (3) crevasse splays; (4) bioturbated/pedoturbated floodplain; and (5) desiccated playa/floodplain. Braided channel sandstones comprise c. 66% of the examined sequences. Detailed petrographic examination reveals that most of the sandstones are subfeldspathic, feldspathic or sublithic arenites with rarer quartz of lithic arenites. Detrital minerals comprise quartz, K-feldspar, plagioclase, mica, heavy minerals and rock fragments. Authigenic components include silica, feldspar, carbonates, pyrite, anatase, illite, chlorite, corrensite, kaolinite and illite-smectite. Within the total sample suite there are a significant number of clay-prone samples in which the clays are both detrital and authigenic in origin. An extensive ICP-AES analysis for 29 elements was performed on 563 samples. The results allow the clay-prone material to be distinguished from the sandstones owing to higher levels of Al 2 O 3 , MgO, K 2 O, TiO 2 and many trace elements (including Li, La and other transition metals) associated with the clays. Since samples were obtained only from cored intervals from the upper parts of the Triassic section, a complete stratigraphic subdivision and correlation for the study area was not attempted. Bivariate analyses of the ICP-AES data reveal diagnostic correlations between many of the major and trace elements. Moreover, excellent separation between samples from the different wells was seen on a triangular plot of Na 2 O, Fe 2 O 3 + MgO and K 2 O. This separation was even more clearly visible on a triangular plot of Li, Zn and Ni. This appears to be the first use of such a plot for distinguishing Triassic sanstone sequences, and it may yet prove applicable in other intervals. The chemical data also shows stratigraphic differences within some of the Triassic sequences examined. Two of the more extensively cored wells have clearly identifiable chemical signatures for the upper and lower parts of the sequences examined, which can be attributed to rapidly shifting sources and transport regimes during the deposition of these sediments. The extent of this chemical distinction is currently being investigated. Discriminant analysis of the ICP-AES data from the sand-prone samples permitted an excellent confirmation of the distinction between the chemical signatures for each of the wells studied. Discrimination between facies using data from the clay prone samples was not attempted owing to the limited availability of data. However, sand- and clay-prone facies can be broadly distinguished on the basis of their bulk chemistry.